LOUD cold start knock ... goes away in 20-30 seconds after startup

The "leak after changing to synthetic oil" is a holdover from at least 20 years ago. That will not be a problem with your truck, or any newer vehicle. If you have a vehicle from the 70's or earlier, I would not recommend the switch, though.

Though it's too late, but it's called piston slap, or morning sickness. Cylinder bores are slightly oval, due to improper machining, and piston skirts slap on them on cold engines, until engine warms up and metal expands.
Seafoam does not help much, I tried, and I use Seafoam on everything.
GM issues a fancy multi page letter explaining how normal it is and other bs. Somehow, it'sa not normal on Toyota or Nissan trucks.
The only real cure to it, as there is no way, metal slapping on metal will NOT eventually harm something, is to pull engine out, re-bore cylinders, and re-set pistons.
For what it is, mine been slapping for the last 50 or so thousand miles. THat's why it's sitting at dealer on consignment now. It just ain't right, and I do not want to deal with after effects.

[MENTION=50075]SurrealOne[/MENTION] can you take a video of the noise so we can hear if its a knock or tick because they are very different. Tick is usually lifters which all push rod engines can experience especially ones with hydraulic lifters. Chevy engines have a long history making this noise when they have dirty/old oil and an aftermarket filter.

A Knock can be much worse and may indicate improper wear, piston slap, or misfires. So really all of us are just guessing.

Sadly, it did it a few days in a row and then seems to have stopped, completely. The days it did it was when another cold front came through and it was unusually cold (and rainy) for this time of year, here. (40's in May is unusual, here.) It's definitely NOT a tick, it's a loud knock that seems to last only until the truck gets her fluids moving around.

I talked to my local mechanic (one of two I will let touch my truck -- a man I've known for years), last week, and he said change the filter to an AC Delco filter at the next oil change and call it a day -- that this is, indeed, common with our engines and that the only real fix is to overhaul or replace the engine if the filter's not the cause.

I'll eventually do that (overhaul the engine) ... but not until the current engine dies. Until then, it is what it is.

Sadly, it did it a few days in a row and then seems to have stopped, completely. The days it did it was when another cold front came through and it was unusually cold (and rainy) for this time of year, here. (40's in May is unusual, here.) It's definitely NOT a tick, it's a loud knock that seems to last only until the truck gets her fluids moving around.

I talked to my local mechanic (one of two I will let touch my truck -- a man I've known for years), last week, and he said change the filter to an AC Delco filter at the next oil change and call it a day -- that this is, indeed, common with our engines and that the only real fix is to overhaul or replace the engine if the filter's not the cause.

I'll eventually do that (overhaul the engine) ... but not until the current engine dies. Until then, it is what it is.

@SurrealOne can you take a video of the noise so we can hear if its a knock or tick because they are very different. Tick is usually lifters which all push rod engines can experience especially ones with hydraulic lifters. Chevy engines have a long history making this noise when they have dirty/old oil and an aftermarket filter.

A Knock can be much worse and may indicate improper wear, piston slap, or misfires. So really all of us are just guessing.

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I know the noise that surrealone is referring to, many of us do. It is much different than a tick, it is a knock. You can actually tel that it is not originating from the head area. Mine sounded much lower than the lifter tick. My 2002 did it if I started it when it was cold and drove it before letting it warm up .

You can actually tel that it is not originating from the head area. Mine sounded much lower than the lifter tick. My 2002 did it if I started it when it was cold and drove it before letting it warm up .

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Yea, that's what creeped me out. I wasn't willing to move the truck unless it went away, which it did. That's when I began reading about piston slap, lifter tick, loose torque converters, and people saying GM told them that the first two of those were 'normal'.

It is what it is. It seems to be gone, for the moment, but I expect it to return when the weather cools down again, in the fall/winter.

Glad, that you made the conclusion that satisfies you! A rebuilt motor is cheaper than a "New Lesser Truck"...........

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Heh, I long ago decided that anything that breaks is just an upgrade opportunity. I have no plans to sell or scrap my truck unless I have absolutely no choice in the matter. (For instance, if I had to choose between my truck and eating, I'd choose my truck, erm, I mean eating. )

What has happened several times in very cold weather (with a V8 but not a Chevy) is that I've needed to start and just barely move the vehicle so that it didn't even run a minute, and when I restarted the vehicle- could have been hours later- it sounded like there was no oil pressure and like there was a horrible rod knock. It would clatter like crazy if I revved the engine at all. This was a low mileage 2004 Infiniti M45, the first generation. I think that the cold oil stayed up in the (big DOHC) cylinder heads and that what I was hearing actually was a rod knocking from lack of oil. Once the engine warmed a little the noise went away; I assume the warm oil flowed back into the pan and got circulated by the pump. The oil pressure warning light never came on, though. I never shut off a cold engine in that car after that happened a couple times.